The invention relate to die casting of lost foam patterns for use in casting an engine block for a two stroke engine. The invention also relates to die casting of such engine blocks which include a cylinder bore and a helical transfer passage which, because of the helical or inclined relation to the axis of the cylinder, includes an undercut portion which generally prevents one-piece die member removal in a direction along the axis of the cylinder. Still further, the invention relates to die members for forming such transfer passages.
In the past, formations with under cut areas were formed by a core which, in general, formed a major portion of the formation including the under cut area and which was pulled or removed entirely from the die cast lost foam pattern along an axis extending parallel to the direction of the under cut and at a slight angle to the axis of the supporting die member before pulling or withdrawal of the supporting die member. Such removal involved a relatively long stroke length which adds to the mold size, and to the precision of core location, and to the cost of the die assembly, as well as increasing the die casting cycle time.
Attention is directed to the following prior U.S. Pat. Nos.:
______________________________________ 4,243,093 Neiman, J. R. January 6, 1981 4,632,169 Osborn, H. L. December 30, 1986 4,640,333 Martin, et al. February 3, 1987 4,657,063 Morris, R. L. April 14, 1987 4,691,754 Trumbauer, et al. September 8, 1987 4,721,149 Hesterberg, et al. January 26, 1988 Re.31,488 Trumbauer January 10, 1984 ______________________________________
Particular attention is directed to the Osborn U.S. Pat. No. 4,632,169 which discloses a three piece lost foam pattern. In this reference, the transfer passage portion in the middle piece identified by the numeral 18 in FIG. 2 of the Osborn patent does not extend helically with respect to the cylinder axis.